I have found that a few people could not install Visual Web Developer 2008 or Visual Studio 2008 because Microsoft Web Authoring Component setup was failing on their machine. In order to diagnose the problem it is necessary to look into SetupExe(...).log files that can be located in the user TEMP folder. The file is long, but close to the end one can find actual error message. Often it is about some Office 2007 Beta component. Remember, designer is based on Office 2007 so its setup will fail when Office 2007 pre-release bits are installed. However, in some cases installed product did not appear in Control Panel. However, at the end of the SetupExe(...).log file one could find 

Catalyst beta product conflict check failed. Office Beta product conflict is detected on the computer, productcode={30120000-00B2-0409-0000-0000000FF1CE} Not showing message because suppress modal has been set.  Title: 'Setup Errors', Message: 'Setup is unable to proceed due to the following error(s): The 2007 Microsoft Office system does not support upgrading from a prerelease version of the 2007 Microsoft Office system. You must first uninstall any prerelease versions of the 2007 Microsoft Office system products and associated technologies. Correct the issue(s) listed above and re-run setup.

Quick Web search for 30120000-0020-0409-0000-0000000FF1CE releaved that is was Compatibility Pack for the 2007 Office system (Beta)". Look here for more details: http://www.brokenwire.net/bw/Programming/58/visual-studio-2008-installfest-problems-with-the-visual-web-developer

Compat pack can be uninstalled by running

 

msiexec /x {30120000-0020-0409-0000-0000000FF1CE}

 

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